1. Field of the Invention
The present invention relates to the method and apparatus for recovering energy during the pressure let-down of high pressure reactor effluent.
2. Prior Art
Many chemical reactions are conducted under conditions of high pressure. At some point in the process, this pressure is relieved or dissipated so that the product, unreacted components, etc., can be recovered. Not infrequently, considerable energy has been put into pressuring the system and reactants. The conventional manner of operating such systems has been merely to lose the energy represented by the effluent pressure by reducing the pressure across a valve.
Most of the prior effort to recover this "process energy" has been concentrated on the design of a turbine through which the reaction effluents would be passed, as shown, for example, in U.S. Pat. Nos. 2,850,361 and 3,649,208. Such an approach may work in a single phase reaction system, however, in a multiphase system, particularly those wherein the pressure reduction is employed to cause phase separation, a turbine is generally unsatisfactory. Many difficulties exist in design of such a turbine, because as the pressure is reduced, a liquid or solid phase separates from the gas and tends to coat turbine blades and plug passages. Turbine construction is such that imbalancing of the blades by random deposition of material thereon can cause failure of the engine.
In the more specific case of a system for recovery of process energy from the high pressure, low density polyethylene process, care must be taken that the polyethylene is not trapped in a dead spot, e.g., on a turbine blade, for a sufficient length of time to cross link or form degradation products which may then find their way to the product with the result being lost in product consistency and quality.
It is a feature of the present invention that a substantial portion of the process effluent energy is recovered. It is an advantage of the present system that the energy recovery apparatus is not as likely to become inoperative due to fouling as the prior art turbine systems.
It is a further feature of the present invention that energy is recovered by isentropic expansion of the reactor (reaction) effluent as opposed to the isenthalpic and irreversible expansion of the reactor effluent across a valve.